When reproducing a digital audio signal with a conventional speaker, the digital audio signal is first converted into an analog audio signal via a digital low-pass filter by a digital/analog converter (hereinafter referred to as D/A converter). Next, the analog audio signal is amplified via an analog low-pass filter by an amplification circuit with variable gain, and is then input to the voice coil of the speaker.
In the speaker, a DC magnetic field is produced all the time at the magnetic gap in a magnetic circuit, including a yoke with a magnet pole and a plate. Therefore, when the analog audio signal is input to the voice coil that is placed to cross the DC magnetic field, a diaphragm is driven by force generated according to the Fleming's left-hand rule.
With this driving method, however, a D/A converter for converting a digital audio signal into an analog audio signal is essential. In the case of using the D/A converter, in order to obtain a satisfactory analog signal, a digital low-pass filter of an over-sampling type and a higher-order analog filter must be installed before and after the D/A converter, respectively. This method, therefore, causes the structure of the speaker driving circuit to become complicated and the manufacturing costs to increase.
Moreover, to drive the diaphragm of the speaker, the analog signal obtained by D/A conversion must be amplified with an amplification circuit. The amplification circuit produces a large amount of heat during amplification. Hence, problems arise, namely, the operations of circuits other than the amplification circuit become unstable and the consumption of power increases.